This invention relates generally to fluid separators and in particular to a stack of tubular solid oxide fuel cells.
In general, a solid oxide fuel cell (SOFC) comprises a pair of electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of about 1000xc2x0 C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:
Anode reaction:
H2+O=xe2x86x92H2O+2exe2x88x92
CO+O=xe2x86x92CO2+2exe2x88x92
CH4+4O=xe2x86x922H2O+CO2+8exe2x88x92
Cathode reaction:
O2+4exe2x88x92xe2x86x922O=
Known SOFC designs include planar and tubular fuel cells. Applicant""s own PCT application no. PCT/CA01/00634 discloses a method of producing a tubular fuel cell by electrophoretic deposition (EPD). The fuel cell comprises multiple concentric layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube.
It is also known to arrange a plurality of tubular fuel cells in an array or xe2x80x9cstackxe2x80x9d to increase electrical output. Designs have been proposed for stacking together relatively large-diameter (xe2x89xa72 mm) thick-walled tubular fuel cells that are essentially self-supporting; for example it is known to stack large diameter tubular fuels cells in a grid-like pattern and interconnect the fuel cells with nickel felt spacers. This and other known designs for large diameter self-supporting tubular fuel cells are not particularly well suited for small diameter fuel cells (xe2x89xa62 mm), especially if such small diameter fuel cells are arranged into a tightly-packed array. It is therefore desirable to provide an improved stack design that enables the close-packing of a plurality of small-diameter tubular fuel cells.
According to one aspect of the invention, there is provided a fuel cell stack comprising a plurality of tubular fuel cells embedded in a solid-state electronic or mixed (electronic and ionic) conductive porous matrix. Each fuel cell comprises an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers. A first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells, and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof. The matrix may have a foam-like microstructure, and may have a porosity of between 40 and 95%.
The fuel cells may be of the solid-oxide type and in such case the matrix composition may include an electronic or mixed conductive material. In particular, the matrix material may be lanthanum strontium manganate. The diameter of at least one of the fuel cells may be in the range of about 10 xcexcm to 2000 xcexcm. The inner electrode layer may be an anode and the outer electrode layer a cathode, and in such case, the first reactant is oxidant and the second reactant is fuel. The inner electrode layer of at least one the fuel cells may be produced by one of electrophoretic deposition, metal electrodeposition, or composite electrodeposition.
According to another aspect of the invention, there is provided a method of producing a fuel cell stack that comprises:
(a) producing a plurality of tubular fuel cells, each fuel cell having an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers;
(b) coating the fuel cells with a slurry having a composition that includes a matrix material that upon sintering, becomes a solid-state electronic or mixed (electronic and ionic) conductive porous matrix;
(c) stacking the fuel cells such that the slurry coating of each fuel cell is in contact with the slurry coating of adjacent fuel cells; and
(d) sintering the coated and stacked fuel cells to solidify the matrix and embed the fuel cells therein,
thereby producing a stack wherein a first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells, and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof.
The step of producing the fuel cell may comprise first forming an inner electrode layer on a combustible deposition cathode by one of electrophoretic deposition, metal electrodeposition, or composite electrodeposition, then forming an electrolyte layer on the inner electrode layer by electrophoretic deposition, then forming an outer electrode layer onto the electrolyte layer, and then applying a sintering step that combusts the deposition cathode, thereby leaving a hollow tubular fuel cell.
The matrix material in the slurry may be one in the group of lanthanum strontium manganate, doped LaCrO3 (La1xe2x88x92xSrxCr03, La1xe2x88x92xCaxCr03, La1xe2x88x92xMgxCr03, LaCr(Mg)03, LaCa1xe2x88x92xCry03, LaCr(Mg)O3, stainless steel (316, 316L), cermet (such as Ni-Yittria stabilized zirconia or any Ni and doped zirconia cermet, Ni dopedxe2x80x94Ce02 cermet, Cu doped-ceria cermet), silver and its alloys, Inconel steel or any super alloy, or ferritic steel, SiC, MoSi2. The slurry may further include a foaming agent, such that upon a selected heat treatment, a solid-state porous matrix is formed having a foam-like microstructure. The slurry may also or instead include combustible particles, such that upon a selected heat treatment, a solid-state porous matrix is formed having a porous microstructure.
The steps of coating the fuel cells with slurry and stacking the fuel cells may comprise stacking the fuel cells in a container, then adding the slurry into the container such that the fuel cells in the container are immersed in the slurry. Alternatively, the steps of coating the fuel cells with slurry and stacking the fuel cells comprise coating each fuel cell then placing combustible spacers between the fuel cells before stacking. Yet another alternative approach comprises coating the fuel cells then placing the coated fuel cells on a flexible sheet, then manipulating the sheet such that the fuel cells are arranged into a desired stack configuration.
According to yet another aspect of the invention, there is provided a method of producing a fuel cell stack that comprises:
(a) producing a plurality of tubular fuel cells, each fuel cell having an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers;
(b) providing a container of combustible members in a desired stack configuration and immersed in a slurry having a composition that includes a matrix material that upon sintering, becomes a solid-state electronic or mixed (ionic and electronic) conductive porous matrix;
(c) sintering the slurry and combustible members such that the matrix is formed and the combustible members combust, thereby producing a plurality of channels in the matrix; and,
(d) inserting at least one fuel cell into at least one channel;
thereby producing a stack wherein a first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells, and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof. This method may include the further step of (e) adding slurry into the channel between the fuel cell and the matrix, then sintering the slurry such that the fuel cell is securely embedded in the matrix.
According to yet another aspect of the invention, there is provided a method of producing a fuel cell stack that comprises:
(a) producing a plurality of tubular fuel cells, each fuel cell having an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers;
(b) embedding the fuel cells in a combustible template material;
(c) impregnating the template material with a slurry having a composition that includes a matrix material that upon sintering, becomes a solid-state electronic or mixed (ionic and electronic) conductive porous matrix; and,
(d) sintering the impregnated and fuel cell embedded template material such that the template material combusts, and the matrix is formed;
thereby producing a stack wherein a first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells, and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof. The template material may be one in the group of a sponge, carbon felt, or graphite felt.
According to yet another aspect of the invention, there is provided a fluid separation apparatus that comprises a plurality of tubular fluid separation membrane assemblies and a solid-state porous matrix in which the assemblies are embedded. Each assembly comprises a porous separation layer and a porous support layer in adjacent contact with the separation layer. The porosity of the separation layer is selected according to the fluids to be separated. An unseparated fluid is flowable through one of the matrix or the inside of at least one of the assemblies, and a separated fluid separated from the unseparated fluid by the separation layer is flowable through the other of the matrix and the inside of at least one of the assemblies.
The separation layer may have a thickness of between about 0.5 to 30 xcexcm, a porosity of less than or equal to 0.3 xcexcm, and a composition that includes one or more of Al2O3, zirconia, Al2O3xe2x80x94zirconia composites or TiO2. The porosity of the support layer may be greater than or equal to 0.3 xcexcm, and may have a composition that includes one or more of Al2O3, zirconia, Al2O3xe2x80x94zirconia composites, clay, or TiO2.
The matrix may have a foam-like microstructure and a composition that includes one or more of Al2O3, zirconia, Al2O3xe2x80x94zirconia composites or steel. The matrix may also be coated with TiO2 photo catalyst.
The separation layer may serve as a membrane reactor and thus have a composition that includes material that affects the conversion or selectivity of one or more chemical reactions of the fluids flowable through the apparatus. In particular, the membrane reactor layer may have a composition that includes Pd or Pd-alloy, and a thickness of between about 0.5 and 10 xcexcm.